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Re: Entailment / Datalog Complete, Sound, Terminating / English Reasoning

From: Graham Klyne <GK@ninebynine.org>
Date: Fri, 23 May 2003 10:40:30 +0100
Message-Id: <>
To: Adrian Walker <adrianw@snet.net>
Cc: www-rdf-logic@w3.org

At 22:12 22/05/03 -0400, Adrian Walker wrote:
>I was surprised to learn that RDF reasoning appears to be very restricted, 
>as in:
>  "...a more complex fact is expressed in RDF using a conjunction 
> (logical-AND) of simple binary relationships. RDF does not provide means 
> to express negation (NOT) or disjunction (OR). The expressive power of 
> RDF corresponds to the existential-conjunctive (EC) subset of first order 
> logic"   [http://www.w3.org/TR/rdf-concepts/]
>Surely, for RDF to function as useful service-finding glue in the Semantic 
>Web, it should at least support discovery of resources by being able to 
>walk arbitrary depth first- and second-order hierarchies/ontologies ?

[I should mention that the wording you cite is not strictly accurate, and 
is being changed, but for the purposes of this discussion may be regarded 
as not-too-misleading.]

I'd agree that such capabilities may be required in the tools and systems 
that use RDF information, but I don't think that's equivalent to saying 
that the capabilities must be built into RDF itself.  Any more than, say, 
bare numbers contain the means to express addition or multiplication.

There is work in progress to address the other capabilities you 
seek:  informally, there's CWM and a number of other software developments 
that use RDF.  And in the standards arena, there's the W3C OWL working 
group.  And there are groups of people looking into query and rules 
languages for RDF, in a pre-standardization mode.

In a sense, the rest of your message hints at the difficulty, in which you 
mention at least two possible (different?) approaches to reasoning that 
might be used with RDF.  Do we really know enough at this stage to know 
what form web-based reasoning should take?  Personally, I think this is an 
area where different mechanisms will be appropriate for different purposes, 
varying from utterly ad-hoc programs that manipulate certain RDF 
vocabularies (e.g. PIMs using RDF to exchange schedule information) to very 
formalized systems for establishing and interpreting digital contracts 
(e.g. access control to sensitive facilities).  I think the strength of RDF 
here is that it's agnostic enough about reasoning methods to be used with 
any or all of these -- it lets us exchange information, but doesn't tell us 
how to "think".

And meanwhile, various folks are working away at creating a diverse range 
of actual applications using RDF, notwithstanding it's very limited 
expressive capabilties...


Graham Klyne
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E
Received on Friday, 23 May 2003 07:06:00 UTC

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